Liquid crystal display devices (LCDs) are display devices including a liquid crystal substance injected between two glass substrates and using the electro-optical properties of the liquid crystal under the application of external voltage. Such LCDs are distinguished from other known display devices in that they use external incident light. In addition, LCDs are advantageous in that they are thin and light and consume less power. In general, LCDs have been widely used for various applications, including televisions (TV), monitors for personal computers (PC), car navigations, digital cameras, cellular phones, or the like.
The driving modes of such liquid crystal displays include twisted nematic (TN) modes using nematic crystals, super twisted nematic (STN) modes, in-plane switching (IPS) modes, vertical alignment (VA) modes, optically compensated bend (OCB) modes, or the like. Since the liquid crystals having such types of driving modes are optically anisotropic materials having two different indexes, i.e., ordinary refractive index and extraordinary refractive index, light path and birefringence are varied with the angle of incidence of light. Therefore, variations in contrast ratios (CR) and gray scale inversion phenomena occur depending on the direction of watching a screen. Particularly, because vertically aligned liquid crystals, such as twisted nematic liquid crystals, show a large variation in phase difference (And) depending on the direction of light propagation, they have disadvantages of a narrow view angle and a low contrast ratio. To improve such disadvantages, compensation films have been used to compensate for such a phase difference and to increase a view angle.
In this context, Japanese Laid-Open Patent No. 2004-240012 discloses a retardation sheet including a first optical anisotropic layer having a phase difference of substantially λ/2(π) at a wavelength of 550 nm, a second optical anisotropic layer having a phase difference of substantially λ/4(π/2) at a wavelength of 550 nm, and a third optical anisotropic layer having alternating anisotropy, stacked successively, and has a retardation/wavelength value of 0.2-0.3 as measured at a wavelength of 450 nm, 550 nm and 650 nm, wherein at least one of the first optical anisotropic layer and the second optical anisotropic layer is formed of liquid crystal molecules immobilized with nematic alignment in which the liquid crystal molecules have a tilt angle of 5-35°. However, such a stacked sheet requires a complicated treating process, such as adhesive coating or binding, and may generate defected products in the case of a misaligned axis, resulting in a drop of yield and an increase in manufacturing cost. In addition, because expression of individual optical anisotropy may be affected by various conditions, including polymer molecular weights, temperatures or extension rates, it is difficult to control the anisotropy of each layer precisely.
To solve the above problems, Japanese Laid-Open Patent No. 2005-208414 discloses a high-dispersion retardation film including liquid crystal monomers having a discotic mesogen and a nematic mesogen end-capped with a polymerizable group in the molecules, wherein the discotic mesogen and the nematic mesogen are aligned in such a manner that both optical axes are substantially in parallel with each other and are immobilized while maintaining such alignment. Japanese Laid-Open Patent No. 2006-78670 discloses an integral optical compensation sheet having an optical anisotropic layer having different refraction indexes in three directions and a transparent support, wherein the optical anisotropic layer is formed by polymerizing discotic liquid crystals having at least one polymerizable group and a compound having multiple polymerizable groups. In addition, Japanese Patent Publication No. 3399705 and No. 2587398 disclose a method of realizing an effect of view angle compensation through hybrid alignment of discotic nematic liquid crystals.
The inventors of the present invention have conducted many studies to provide an effect of view angle compensation in an optical compensation film. We have found that it is possible to provide an optical compensation film for LCDs having a significantly improved effect of view angle compensation by using a novel nematic liquid crystal compound having at least two mesogens linked to each other with a constant angle and by aligning the liquid crystals in the form of hybrid alignment.